In a synchronous direct-sequence code division multiple access (S-CDMA) system, users communicate simultaneously using the same frequency band via orthogonal modulation or spread spectrum. The number of orthogonal spreading codes (>1) limits the total capacity of the system. To increase the capacity of a CDMA system in a given service area, without requiring additional frequency bandwidth, space division multiple access (SDMA) can be employed.
Sectorization is the most common approach to providing space division multiple access (SDMA). Simply stated, sectorization splits the coverage area into multiple sectors, with each sector being serviced by independent antenna beams.
A SDMA/CDMA system employing sectorization has a user capacity that scales linearly with the number of sectors, but requires users along sector boundaries to be frequency isolated and, thus, additional frequency bandwidth is needed. That is, frequency division is typically employed along the adjacent sector boundaries, which requires additional frequency spectrum. Sectorization also suffers from decreased Erlang efficiency due to the limited capacity per sector. That is, the total supported traffic of a sectored cell is equal to the sum of the supported traffic in the individual sectors. However, the sum of the supported sector traffic is typically much less than could be supported in a cell with the same total number of channels, but with no sector constraints. As an example, a system with P=16 channels per sector and M=4 sectors can support 100 users per sector, or a total of 400 users per cell, assuming that GOS=0.02 and a traffic density of 0.1 Erlangs. A system with the same number of total channels (MP=64), but with no sector constraints, can support approximately 550 users under same assumptions. As such, since sectorization places restrictions on the locations of users, it is not the most efficient approach to supporting communications within a given area.
The concepts of SDMA and adaptive antennas has been actively researched. The use of SDMA and adaptive antenna systems in the context of CDMA has also been investigated. For example, reference can be had to “Smart Antennas for Wireless Communications: IS-95 & Third Generation CDMA Applications”, by J. C. Liberti and T. S. Rappaport, Prentice Hall, 1999. However, this text deals primarily with mobile CDMA applications, where smart antenna systems are used to provide isolation in asynchronous or quasi-synchronous CDMA systems. The text suggests nulling interfering users (interferers), but does not suggest that the interferers have the same CDMA spreading code for providing increased system capacity.
The use of antenna systems in conjunction with CDMA is also considered in U.S. Pat. No. 4,901,307. In this patent antenna systems are used to point directive beams or to create interference patterns of “maximum signal to noise” at a given receiver location, and requires knowledge of the receiver location or the physical channel between the transmitter and the receiver. This patent also does not consider the S-CDMA case, nor the possibility of reusing the orthogonal spreading codes within the same coverage area.
Conventional practice (see FIG. 7) also performs spatial filtering prior to despreading. Reference can be made to J. Liberti and T. Rappaport, Smart Antennas for Wireless Communications: IS-95 & Third Generation CDMA Applications, Prentice-Hal, Upper Saddle River, N.J., 1999, where the idea of despreading the signal prior to spatial filtering is mentioned in the context of an asynchronous CDMA reverse link. However, the authors claim that the other technique (spatial filtering before despreading) is preferred, as it requires only “one despreading module for each spatial filtering receiver. If we reverse the order of spatial processing and despreading, then M despreaders are required for each spatial filtering receiver.” This is currently the accepted view of those working in the field.
For a fixed wireless local loop (FWL) application, in which a base station provides telephone and data service to a 360 degree cell, a circular antenna array is a very attractive antenna configuration. It is possible to split the cell into sectors and service each sector with a linear antenna array, however this approach has the disadvantage of having sector boundaries such that users that are located on a boundary between two sectors will provide a substantial amount of interference to the adjacent sector. In order to eliminate explicit sector boundaries, the use of SDMA with a circular antenna array permits one base station to service 360 degrees.
A review of the literature on SDMA finds that it is common to initially assume that antenna arrays may be implemented with an arbitrary element spacing, but then to go on to make the simplifying assumption that the elements are spaced a half-wavelength apart. However, when operating with frequencies in, by example, the 2–4 GHZ range the carrier wavelength is between 7.5 and 15 centimeters. This implies that the antenna elements need to be very close together if half-wavelength spacing is desired. For example, a system operating at 3 GHz has a carrier wavelength of 10 centimeters. If one half wavelength antenna element spacing is desired in a 16 element array, then the diameter of the circular array is approximately 25 cm. Referring to the FIG. 11A, it can be seen that a 25 cm diameter cylinder of antenna elements will need to reside on top of a tower if it is to service a cell and have a 360 degree view. It can be appreciated that since tower rental fees increase the higher on the tower one positions an antenna, the rental fees required for the 25 cm diameter antenna array will be considerably more than for an antenna array positioned lower on the tower. This assumes that there is space available at or near the top of the tower, which may not always be the case.